Use flexmembers on IBM XL C for AIX

This removes a workaround where Emacs did not use flexible
array members when compiled with IBM XL C.  Instead, avoid
the problem by making the aliasing issues more obvious to
this compiler.
* admin/merge-gnulib: Don’t remove m4/flexmember.m4.
* m4/flexmember.m4: Copy from gnulib.
* configure.ac (AC_C_FLEXIBLE_ARRAY_MEMBER): Remove workaround.
* src/alloc.c (allocate_string_data): Rephrase to avoid aliasing
problem that would otherwise mess up code generated for flexible
array members by IBM XL C for AIX, V12.1.
* src/conf_post.h (FLEXIBLE_ARRAY_MEMBER): Remove; now done
by gnulib code.

2 files changed, 9 insertions, 17 deletions

diff --git a/src/alloc.c b/src/alloc.c
index 1092a34801a..41b2f9e77d2 100644
--- a/src/alloc.c
+++ b/src/alloc.c
@@ -2002,9 +2002,9 @@ allocate_string_data (struct Lisp_String *s,
         mallopt (M_MMAP_MAX, MMAP_MAX_AREAS);
 #endif
 
-      b->next_free = b->data;
-      b->data[0].string = NULL;
+      data = b->data;
       b->next = large_sblocks;
+      b->next_free = data;
       large_sblocks = b;
     }
   else if (current_sblock == NULL
@@ -2014,9 +2014,9 @@ allocate_string_data (struct Lisp_String *s,
     {
       /* Not enough room in the current sblock.  */
       b = lisp_malloc (SBLOCK_SIZE, MEM_TYPE_NON_LISP);
-      b->next_free = b->data;
-      b->data[0].string = NULL;
+      data = b->data;
       b->next = NULL;
+      b->next_free = data;
 
       if (current_sblock)
         current_sblock->next = b;
@@ -2025,14 +2025,16 @@ allocate_string_data (struct Lisp_String *s,
       current_sblock = b;
     }
   else
-    b = current_sblock;
+    {
+      b = current_sblock;
+      data = b->next_free;
+    }
 
-  data = b->next_free;
+  data->string = s;
   b->next_free = (sdata *) ((char *) data + needed + GC_STRING_EXTRA);
 
   MALLOC_UNBLOCK_INPUT;
 
-  data->string = s;
   s->data = SDATA_DATA (data);
 #ifdef GC_CHECK_STRING_BYTES
   SDATA_NBYTES (data) = nbytes;
diff --git a/src/conf_post.h b/src/conf_post.h
index 865d0183a57..6d54524b970 100644
--- a/src/conf_post.h
+++ b/src/conf_post.h
@@ -358,16 +358,6 @@ extern int emacs_setenv_TZ (char const *);
 #define INLINE_HEADER_BEGIN _GL_INLINE_HEADER_BEGIN
 #define INLINE_HEADER_END _GL_INLINE_HEADER_END
 
-/* To use the struct hack with N elements, declare the struct like this:
-     struct s { ...; t name[FLEXIBLE_ARRAY_MEMBER]; };
-   and allocate (offsetof (struct s, name) + N * sizeof (t)) bytes.
-   IBM xlc 12.1 claims to do C99 but mishandles flexible array members.  */
-#ifdef __IBMC__
-# define FLEXIBLE_ARRAY_MEMBER 1
-#else
-# define FLEXIBLE_ARRAY_MEMBER
-#endif
-
 /* 'int x UNINIT;' is equivalent to 'int x;', except it cajoles GCC
    into not warning incorrectly about use of an uninitialized variable.  */
 #if defined GCC_LINT || defined lint